I'm a Bizenghast fan, but not a die-hard one. But that being said, I thoroughly enjoyed the prose version of Bizenghast.
Here's what I look for in a novellization of a manga, and here's how Bizenghast stacksup. (only the most minor of spoilers, I promise)
IT HAS NEW INSIGHTS INTO THE ORIGINAL EXPERIENCE. The biggest advantage that the prose has over the original version is that it allows us to hear the inner voices of Dinah and Vincent (and to a lesser extent, Edrear and Aunt Jane), which allows for a more complete insight into what the characters are going through in the original.
IT MATCHES CONTINUITY. Nothing in this book contradicts what occurs in the Manga. That's important to me, because when continuity is ignored, it takes me out of the experience. The author made sure that everything matches up with the original.
IT ADDRESSES SOME THINGS THAT CONFUSED ME. I love Dinah. Don't get me wrong. But she's becomes almost a completely different character between the first two Manga books. By shifting to a more internal view of Dinah, I got to see how those changes occurred in a satisfying manner. Also, some of the things from the Manga that I only "sort of" got (like the rescue of Cagey) made a lot more sense in this format.
IT HAS SOME NEW STUFF. I loved the new ghosts, and there was a chapter with the Four Tower Guards that i just LOVED! Some very cool insights into Edrear as well.
IT's WELL WRITTEN. There's definitely an homage to Lewis Carroll in the writing style of the book, which works very well considering that's the source material the original creator drew upon. Thorgersen matches LeGraw's episodic nature of the original very nicely.
IT'S FUN. Of all the categories, this was the most important for me. I had a good time reading this. If you're a Bizenghast fan, die-hard or not, you'll enjoy it.